The invention relates to
thermoplastic molding compositions comprising
                A) from 29 to 97.5% by weight of a thermoplastic polyamide,        B) from 1 to 20% by weight of melamine cyanurate,        C) from 0.5 to 10% by weight of an organic phosphorus compound based on 9,10-dihydro-9-oxa-10-phosphaphenanthrene oxide (DOPO) as parent structure,        D) from 1 to 50% by weight of a fibrous filler, the aspect ratio (L/D) of which is from 4 to 25, and the arithmetic average fiber length of which is from 40 to 250 μm, and        E) from 0 to 50% by weight of further additives,where the total of the percentages by weight of components A) to E) is 100%.        
The invention further relates to the use of the molding compositions of the invention for producing fibers, foils, and moldings of any type, and also to the resultant moldings.
Polyamides equipped with fire resistance have recently been gaining importance. Products of particular interest here are those formulated in pale colors for the electrical sector. However, although red phosphorus and halogen compounds combined with synergists are known fire-protection systems they are unsuitable for this application sector. Halogen compounds impair electrical properties such as tracking resistance and dielectric strength. The intrinsic color of red phosphorus prevents its use for pale-colored formulations. DE-A 1694254 recommends addition of melamine to produce polyamides formulated in pale colors, without reinforcement but with flame retardancy. Melamine and melamine salts, e.g. melamine cyanurate, are less effective in glassfiber-reinforced polyamides, and the glow-wire resistance of said products is very low—especially when wall thicknesses are low.
Unreinforced molding compositions generally have higher glow-wire resistance but in contrast have the disadvantage of inadequate mechanical properties, such as stiffness and strength. Addition of glass fibers to polyamide mixtures with melamine cyanurate improves mechanical properties, but there is a disadvantageous effect on flame retardancy properties, since glass fibers cause drastic impairment of flame retardancy through what is known as the wicking effect. Correspondingly, EP-A 241 702 discloses that PA mixtures made of glass fibers with melamine cyanurate can be given improved flame retardancy performance by using short glass fibers without size in the mixture.
EP-A 848 729 discloses melamine cyanurate and short glass fibers with particular length distribution.
WO 2008/119693 and 2008/132111 disclose DOPO derivatives as flame retardants.
The effectiveness of flame retardancy additive mixtures is in essence described via fire tests to UL 94-V. However, for particular applications of flame-retardant polymers in installation work within buildings, and also in low-voltage switchgear, it is mainly the glow-wire test to IEC 60695-2-12 that is important, high flame retardancy also being desirable.
To the extent that glass fibers are used in the abovementioned specifications, they can be conventional continuous-filament fibers (rovings) or chopped fibers (fiber bundles of length from 4 to 6 mm). Shear in the extruder then gives a glassfiber length distribution in the product which is about 250-300 μm for conventional processing (based on a product with 25% glassfiber content). A factor that has to be considered here is that the average fiber length generally falls as fiber content increases, since this results in an increased level of fiber interactions in the incorporation zone and thus to an increased level of fiber breakage (F. Raumsteiner, R. Theysohn, Comp. Sci. Techn. 23 (1985) 231).
At glassfiber contents greater than 20% by weight, addition of melamine cyanurate as flame retardant usually becomes insufficient for the glow-wire test to be passed at low wall thicknesses.